1. Field of the Invention
The present invention relates to a high frequency semiconductor device.
2. Background Art
In conventional high frequency semiconductor devices, high frequency signal input/output pads are formed on the semiconductor substrate to receive and output high frequency signals (see, e.g., Japanese Patent Laid-Open No. 60-249374).
When a high frequency semiconductor element is mounted in a package, the high frequency signal input/output pads are usually electrically connected to terminals of the package by gold wires having a diameter of approximately 25 μm. At that time, ground potential pads provided on opposite sides of each input/output pad are wire-bonded (to ground) to reduce degradation in the characteristics of the portions of the input/output pads to which the gold wires have been connected.
The high frequency characteristics of the high frequency semiconductor element are measured by pressing probes connected to a high frequency characteristics measuring device against the high frequency signal input/output pads and inputting or receiving high frequency signals. The tip of such a probe includes: a high frequency signal transmission probe for inputting or receiving a high frequency signal; and ground potential probes provided on opposite sides of the high frequency signal transmission probe. On the high frequency semiconductor element, the ground potential pads (provided on opposite sides of each high frequency signal input/output pad) are disposed at positions corresponding to the ground potential probes.
Incidentally, high frequency semiconductor devices typically employ a GaAs substrate having a thickness of approximately 100 μm. Therefore, when a microstrip transmission line is formed on the high frequency semiconductor element, the line width of the high frequency signal transmission line is set to approximately 70 μm to set the characteristic impedance to approximately 50 O. As a result, the line width of the high frequency signal input/output pad connected to the high frequency signal transmission line is also set to approximately 70 μm. (The input/output pad is formed as an extension of the transmission line.)
However, conventional high frequency semiconductor devices having such a structure have a problem in that if gold wires having a diameter of approximately 25 μm are used to wire-bond the high frequency signal input/output pads, only a single gold wire can be connected to each pad since the gold ball portion of the gold wire is approximately 70 μm in diameter. Therefore, as the operational frequency of the high frequency semiconductor element increases, so does the influence of the inductance component of the gold wire in each wire bonding connection portion, resulting in increased high-frequency signal loss. This will degrade the high frequency characteristics.
For example, when a high frequency semiconductor element is mounted in a package using a single gold wire for each high frequency signal input/output pad, the high-frequency signal loss in the wire bonding connection portion for each input/output pad is approximately 0.5 dB at 10 GHz, approximately 2 dB at 30 GHz, and approximately 6 dB at 50 GHz.
The inductance component of the wire bonding connection portion may be effectively reduced by increasing the number of gold wires or increasing the diameter of each gold wire. However, both cases require the line width of the high frequency signal input/output pad to be increased, resulting in a difference in line width between the high frequency signal input/output pad and the high frequency signal transmission line.
A difference between the line widths of the high frequency signal input/output pad and the high frequency signal transmission line results in an impedance mismatch, which increases the reflection loss. For example, when both of them have a line width of 70 μm, the reflection loss is −30 dB or less at 30 GHz. On the other hand, when the line width of the high frequency signal input/output pad is increased to 150 μm (that is, the line width difference is 70 μm), the reflection loss increases to −20 dB at 30 GHz. That is, there is a reflection loss increase of as much as 10 dB.